The present invention relates to a bidirectional photothyristor chip.
Conventionally, as a solid-state relay (hereinafter abbreviated to SSR) to be used with an alternating current, there has been a circuit construction as shown in FIG. 9. This SSR 8 is constituted of a light-fired coupler 3 constructed of a light-emitting device 1 such as an LED (light-emitting diode) and a bidirectional photothyristor 2 for firing, a bidirectional thyristor (hereinafter occasionally referred to as a main thyristor) 4 for actually controlling the load and a snubber circuit 7 constructed of a resistor 5, a capacitor 6 and so on.
An equivalent circuit diagram of the light-fired coupler 3 that constitutes the SSR 8 is shown in FIG. 10. The bidirectional photothyristor 2 is constructed of a photothyristor 9 of CH (channel) 1 and a photothyristor 10 of CH2. The photothyristor 9 of CH1 is constituted by connecting the base of a PNP transistor Q1 to the collector of an NPN transistor Q2 and connecting the collector of the PNP transistor Q1 to the base of the NPN transistor Q2. Likewise, the photothyristor 10 of CH2 is constituted by connecting the base of a PNP transistor Q3 to the collector of an NPN transistor Q4 and connecting the collector of the PNP transistor Q3 to the base of the NPN transistor Q4.
Further, on the CH1 side, the emitter of the PNP transistor Q1 is connected directly to an electrode T1. On the other hand, the emitter and the base of the NPN transistor Q2 are connected directly and via a gate resistor 11, respectively, to an electrode T2. Likewise, on the CH2 side, the emitter of the PNP transistor Q3 is connected directly to the electrode T2. On the other hand, the emitter and the base of the NPN transistor Q4 are connected directly and via a gate resistor 12, respectively, to the electrode T1.
FIG. 11 is a schematic pattern layout of the bidirectional photothyristor 2 of FIG. 10. FIGS. 12A and 12B are schematic sectional views taken along the arrow line A–A′ in FIG. 11. FIG. 12A shows an optically turned-on state, and FIG. 12B shows an optically turned-off state during voltage inversion (during commutation). This bidirectional photothyristor 2 is provided with two anode diffusion regions (P-type) 22 and two P-gate diffusion regions (P-type) 23 on the front surface side of the N-type silicon substrate 21, which are laterally inverted to each other in the figure. A cathode diffusion region (N-type) 24 is provided on the side opposite from the anode diffusion region 22 in each of the P-gate diffusion regions 23. Thus, a PNPN section that constitutes the photothyristor 9 of CH1 in FIG. 10 is formed extended from the anode diffusion region 22 on the right-hand side toward the cathode diffusion region 24 on the left-hand side in the figure. Moreover, a PNPN section that constitutes the photothyristor 10 of CH2 is formed extended from the anode diffusion region 22 on the left-hand side toward the cathode diffusion region 24 on the right-hand side in the figure.
That is, the PNP transistor Q1 on the CH1 side is constructed of the anode diffusion region 22 on the right-hand side, the N-type silicon substrate 21 and the P-gate diffusion region 23 on the left-hand side, while the NPN transistor Q2 on the CH1 side is constructed of the cathode diffusion region 24, the P-gate diffusion region 23 both on the left-hand side and the N-type silicon substrate 21. On the other hand, the PNP transistor Q3 on the CH2 side is constructed of the anode diffusion region 22 on the left-hand side, the N-type silicon substrate 21 and the P-gate diffusion region 23 on the right-hand side, while the NPN transistor Q4 on the CH2 side is constructed of the cathode diffusion region 24, the P-gate diffusion region 23 both on the right-hand side and the N-type silicon substrate 21. The anode diffusion region 22 and the electrode T1 on the right-hand side are connected to each other by an Au wire 25a, while the cathode diffusion region 24 and the electrode T1 are connected to each other via an Al electrode 26 on the right-hand side inside the chip. Moreover, the anode diffusion region 22 and the electrode T2 on the left-hand side are connected to each other by an Au wire 25b, while the cathode diffusion region 24 and the electrode T2 are connected to each other via an Al electrode 26 on the left-hand side inside the chip.
The bidirectional photothyristor 2, which has the aforementioned construction, operates as follows. That is, in FIGS. 10 through 12A, firstly, if the potential polarity is more positive on the electrode T1 side than on the electrode T2 side under the condition that a power voltage higher than the on-state voltage (about 1.5 V) of the device is applied as a bias across the electrode T1 and the electrode T2, then the NPN transistor Q2 on the CH1 side is turned on when the bidirectional photothyristor 2 receives an optical signal from the LED 1. Then, a base current is drawn from the PNP transistor Q1 on the CH1 side, and this PNP transistor Q1 is turned on. Subsequently, a base current is supplied to the NPN transistor Q2 on the CH1 side by a collector current of the PNP transistor Q1, and the PNPN section on the CH1 side is turned on by positive feedback to flow an on-state current corresponding to the load of the AC circuit from the electrode T1 to the electrode T2. In the above case, the positive feedback of the PNPN section does not occur on the CH2 side since the bias applying direction is reversed, and only a primary photoelectric current flows.
On the other hand, if the potential polarity is more positive on the electrode T2 side than on the electrode T1 side, then the PNPN section on the CH2 side is turned on through positive feedback operation quite similarly to the above-mentioned case, and only the primary photoelectric current flows on the CH1 side.
Thus, when the PNPN section on the CH1 side or the PNPN section on the CH2 side performs the firing operation, this current flows into the gate of the main thyristor 4, firing the main thyristor 4. As a prior art reference concerning the bidirectional photothyristor for use in a light-fired coupler as described above, there is, for example, a patent gazette of Japanese Patent Laid-Open Publication No. HEI 10-242449.
In the circuit construction of the SSR 8 shown in FIG. 9, it is the main thyristor 4 that actually controls the load current, and the bidirectional photothyristor 2 is used for optically firing the main thyristor 4. Then, the SSR 8, which has the aforementioned circuit construction, has a feature that it is electrically insulated.
In designing a general SSR device, the bidirectional photothyristor 2 for firing is made to receive light from the LED 1 and become operative with a photoexcitation current of about 10 μA generated at the time. On the other hand, the main thyristor 4 becomes operative with a gate trigger current of about 20 mA, which is the operating current of the bidirectional photothyristor 2. Therefore, the main thyristor 4 cannot be fired at all by the photoexcitation current of the LED 1.
In the case of the aforementioned device, which has the bidirectional channels CH1 and CH2 inside the single chip and is used as a switch for an AC circuit, its commutation characteristic (described in detail later) is an important criterion for evaluating the device. Due to this commutation characteristic, the main thyristor 4 becomes unable to control (turning-off control) the load if it does not have a capacity exceeding the value of the current that is desired to be controlled, disadvantageously resulting in malfunctioning. Likewise, the bidirectional photothyristor 2 is to also malfunction due to the commutation characteristic if it does not have a capacity exceeding the trigger current of the main thyristor 4, the current value being about 50 mA.
In recent years, the economic environment surrounding the electronic industry has become severer, and there are earnestly demanded cost reduction and improvement of handiness of electronic equipment. In order to cope with the above-mentioned demands, it has been attempted to directly control the load only by the bidirectional photothyristor with the main thyristor 4 eliminated to, for example, reduce the parts count in the conventional SSR that has a construction as shown in FIG. 9.
In the above case, if a bidirectional photothyristor as shown in FIG. 11 is employed as the bidirectional photothyristor 2, then the commutation characteristic of this bidirectional photothyristor becomes the most serious problem. This commutation characteristic is an important design parameter, and the controllable load current is determined by this commutation characteristic.
The aforementioned commutation characteristic is herein described. With regard to the commutation characteristic in the case of normal operation, as shown in FIG. 12A, if the incidence of light disappears in a half cycle period of the alternating current during which the CH1 is on, then the on-state continues due to the current holding property of the PNPN section during this half cycle period. Then, if a shift to the next half cycle occurs as shown in FIG. 12B, then the CH2 is not turned on unless there is incident light. However, if an inductive load exists in the AC circuit that is subjected to switching, then the phase of the on-state voltage is delayed relatively to the phase of the AC voltage applied across the electrode T1 and the electrode T2. Therefore, an AC voltage of the inverted phase has already been applied across the electrode T1 and the electrode T2 at the point of time when the CH1 is turned off. Therefore, a voltage of the inverted phase exhibiting a steep rise is to be applied to the CH2 side at the point of time when the CH1 is turned off.
Therefore, holes 27, which remain in the N-type silicon substrate 21 of the bidirectional photothyristor 2, move to the P-gate diffusion region 23 on the right-hand side as indicated by arrow A before disappearing to thereby turn on the PNP transistor on the CH2 side despite no incident light and to promote the positive feedback on the CH2 side, causing a malfunction (commutation failure) that the CH2 is turned on.
That is, the aforementioned “commutation characteristic” is a characteristic that expresses a maximum operating current value Icom that can be controlled without causing the commutation failure as described above.
When the load is directly controlled only by the bidirectional photothyristor 2 with the main thyristor 4 eliminated in the conventional SSR that has a construction as shown in FIG. 9, there is required a capacity enough to endure a load current of about 0.2 A in terms of the capacity of bidirectional photothyristor 2. However, there is a problem that the main thyristor 4 cannot be eliminated since the commutation characteristic Icom required for the bidirectional photothyristor 2 is not smaller than about 200 mArms in the above case, and the malfunction due to the commutation failure occurs in the bidirectional photothyristor 2, shown in FIG. 11, which normally exhibits the commutation characteristic Icom of about one fifth the value.